It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) prayed five rak'ahs. When he finished with the salam, it was said: O Messenger of Allah, has something been added to the prayer? He said: "No; what is that?" They said: You (peace and blessings be upon him) prayed five rak'ahs. He said: "I am a human being like you; I remember just as you remember, and I forget just as you forget." Then he (peace and blessings be upon him) performed the two prostrations of forgetfulness.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9816